ScienceShot

ScienceShot: Hungry Polar Bears Turn to Seabird Eggs

Polar bears are known for dining on whatever they want, from human garbage to reindeer to berries. But in the lower latitudes of the Canadian Arctic, they primarily prey on ringed seals (Pusa hispida), hunting them from sea ice platforms. Over the past 3 decades, however, the sea ice in this region has progressively broken up earlier than in the past due to climate change. The bears now face 2 months of ice-free habitat. Without seals to eat, the bears have increasingly turned to terrestrial prey, including the eggs of northern common eiders and thick-billed murres, scientists have discovered. Over three summers, from 2010 to 2012, researchers surveyed the birds’ nesting colonies on 230 islands and along more than 1000 kilometers of coastline in Hudson Strait and Northern Hudson Bay Narrows, looking for signs of predators. On 16 of the islands, they spotted 22 polar bears, and evidence of bears, such as feces containing eggshell, on an additional 63 islands, the researchers report online today in the Proceedings of the Royal Society B. They also watched bears eating eggs on numerous occasions (pictured). Overall, they found bears at 34% of the eider colonies, and estimated that the birds lost more eggs to them than to gulls and foxes, their usual nest predators. Murres, which nest on steep and narrow cliff ledges, are not as affected by the bears. The researchers do not yet know how the eiders will fare if the bears continue to feast on their eggs, although they think some colonies may go extinct if the intense predation continues. But their study—and others showing that polar bears are also dining on snow goose eggs and caribou—does support claims that polar bears in areas where the ice breaks up early don’t have enough time to hunt seals and acquire the fat reserves they need to make it through the ice-free season. The vanishing sea ice, the researchers conclude, is causing a cascade of unexpected ecological effects—not just on polar bears but also on seabirds.